Metabasic rocks in the varied group of the Moldanubian zone, southern Bohemia - Their petrology, geochemical character and possible petrogenesis

Abstract

Metabasic rocks form an important constituent of the Chýnov and Český Krumlov units belonging to the Varied Group Moldanubian Zone, south Bohemia). The amphibolites are dominated by amphibolite-facies mineral assemblages of mainly tschermakitic amphibole and plagioclase. Hornblendes show compositional variation with Si ∼ 6.5 apfu Mg/(Mg + Fe) ∼ 0.5 and (Na + K)A ∼ 0.5 apfu. Garnet with clinopyroxene are subordinate and occur in a few samples only. No relics of previous greenschist- or granulite-facies assemblages have been observed, most likely due to the relatively simple metamorphic history. The petrology indicates rather close correlation of the Chýnov and Český Krumlov units. The similarities include presence of dolomite in carbonate bodies, graphite schists, rocks with marialitic scapolite, locally also Ti-andradite (± magnetite, epidote) oxidic assemblages and thin layers of Mn-rich garnet-quartz rocks. However, there is a major difference in the oxidation state. Most Chýnov amphibolites have Fe2O3/FeO = 0.70-1.00 and their protolith probably experienced an early incipient oxidation. Great deal of the parental basalts thus could have been effusive. The Český Krumlov amphibolites have Fe2O3/FeO ≤ 0.4, perhaps because they show much closer association with graphite schists that could have been responsible for the reduction of the adjacent rock units. The dataset is dominated by EMORB-like tholefite basalts. interpreted as having been derived by Early Palaeozoic melting of a strongly depleted mantle source (ε.Nd500 = +8.6 to +9.4; TNdDM= 0.43-0.50 Ga). This argues stoutly against Precambrian age of the Varied Group in south Bohemia. The composition of the remaining samples reflects contamination by upper continental crust εNd500 = +3.1 to +1.3, progressive enrichment in Th, development of a significant negative Nb, and lesser P and Ti anomalies on the NMORB-normalized spiderplots). A much smaller group of amphibolites is characterised by steep REE patterns (LaN/YbN = 5.5-11) and high contents of HFSE (Nb, Ta, Zr and P). It is of a clear OIB affinity, with parental alkali basalt (Nb/Y = 0.7-1.6) generated by a low degree of partial melting of a deep, garnet-bearing asthenospheric: mantle source (ε Nd500 = +4.5 to +6. 1; TNdDM = 0.75-0.83 Ga). Metamorphosed doleritic/gabbroic dykes cutting the Palaeoproterozoic Světlik orthogneiss show rather unradiogenic Nd isotopic composition (εNd5)0 = +0.1 and -3.6; TNdDM = 1.34 and 2.03 Ga). This precludes closed-system crystallization from depleted mantle derived melts in Phanerozoic times. The exact age and nature of their parental magma remain enigmatic but any genetic link with the amphibolites in the structurally overlying Český Krumlov Varied Unit seems ruled out. Overall, the most likely tectonic setting of the magmatism was attenuated lithosphere, subjected to an Early Palaeozoic extension, leading eventually to fragmentation of the northern Gondwana margin. The minor OIB component preserved as alkali basalts as well as some contribution to the EMOR-like basaltic magmas was probably added by a rising mantle plume.